The invention relates to an electrical power distribution installation with isolation monitoring. The document FR-A-2,647,220 describes an electrical power distribution installation which comprises a single three-phase A.C. power supply system with isolated neutral. The power supply system is equipped with a digital overall isolation monitor which inputs an auxiliary A.C. voltage called input voltage, of a frequency generally comprised between 4 and 10 Hertz, to the power supply system.
In the event of an isolation fault occurring, the overall isolation monitor measures, by means of a conventional synchronous demodulation process, the values of the leakage resistance and leakage capacitance which are due to the fault. Determination is achieved by prior measurement of the input voltage and of the leakage current in the leakage impedance due to the fault.
An improvement of the device consists in providing, on each branch line, or feeder, of the three-phase power supply system, a current measuring toroid which is connected to a local isolation monitor, called locator. The locator operates in the same way as the overall isolation monitor, i.e. by synchronous demodulation, but the value of the input voltage Ui which it. uses is transmitted to it directly by the overall isolation monitor, whereas the value of-the local leakage current is naturally measured by the measuring toroid or toroids which equip the branch or branches of the feeder.
The overall isolation monitor also transmits to each locator synchronization pulses of the reference sine waves which are used by the synchronous demodulation.
In the case, concerned by the present invention, where the electrotechnical installation comprises several distinct power distribution systems, each having an overall isolation monitor, it is necessary to avoid having a configuration such that several overall isolation monitors are in operation at the same time on the same feeders, which would result in an impossibility of using the monitors. The conventional technique which can be used to overcome this difficulty would consist in providing a set of relays, with extremely complex circuitry, to disconnect certain overall isolation monitors when this proves necessary due to the configurations used. This solution has the shortcoming of being particularly complicated and costly.